Outboard engine units have exhaust ports for discharging exhaust gas from an engine, one example of which is disclosed in Japanese Patent Application Laid-Open Publication No. HEI-07-165181 (JP H07-165181 A).
FIG. 7 is a view explanatory of the outboard engine unit 100 disclosed in JP H07-165181 A. In the disclosed outboard engine unit 100, exhaust gas from an engine passes through an exhaust passage 102, provided vertically within an extension case 101, as indicated by arrow (1) and is then discharged to the outside through an exhaust port 104, provided in a gear case 103, as indicated by arrow (2).
Further, a fallaway or separation region (i.e., vertical region caused by separation of water flows) 106 would occur behind the gear case 103 by water flows produced by a propeller 105. Further, vortex water flows produced by an anti-cavitation plate 107 would join the propeller-produced water flows in the separation region 106 while sucking in the discharged engine exhaust gas. If the vortexes sucked into the separation region 106 are released to the atmosphere, unwanted noise (sound) can occur. Therefore, there has been a need for an outboard engine unit which can reduce unwanted noise.